It's got to be some sort of configuration error, and the obvious suspect is the tyre data, since the speed readings are going to be sourced from the wheel sensors.
If the rolling radius of the wheels as fitted to the +4 is different from those used to calibrate the speedo, then that could be the source of the problem.
I don't suppose you guys have ordered your GDI's with 16" wheels but they've set the cars up with speedos calibrated for 15", or something daft like that?
